Slang
is often particular to a brief period of time, with common usage ranging
from decades to only a few months. Therefore words which are widely
used and understood at one time, do not have the same meanings later.
For example, a good thing may have been "swell" in the 1930s,
1940s and 1950s, "groovy" in the 1960s, and "cool"
in the 1970s.

Because
slang changes quickly it is a language that is difficult
to capture. Also, it is a primarily spoken language, although most of
what endures is written, not spoken.

Slang
serves the important function of identifying people as members of a
group: because we know the same words, we are members of a group and
can therefore speak freely around each other. Slang
also therefore serves to change the level of conversation from formal
to informal, allowing users to handle awkward social situations.